1. Field of the Invention
The present invention relates to a technique for detecting the ejection of ink drops by a printing apparatus.
2. Description of the Related Art
In an ink-jet printer, ink drops are ejected from a plurality of nozzles to print images. The print head of an ink-jet printer is provided with a plurality of nozzles, some of which are occasionally plugged and rendered incapable of discharging ink drops because of an increase in ink viscosity, the entry of gas bubbles, and other factors. Nozzle plugging produces images with missing dots and has an adverse effect on image quality.
Optical detection devices have been proposed for detecting the ejection of ink drops. In such detection devices, the plurality of nozzles mounted on the print head are tested by the mutual movement of the print head and an ink drop detection device. According to these methods, the operating state of each nozzle is determined by a procedure in which the print head is moved, a nozzle is positioned at a specific point, and ink drops are ejected, blocking light from the detection device.
These methods are disadvantageous, however, in that the ink drop detection device and the print head nozzles must be aligned with high accuracy in the direction of main scanning.
Accordingly, an object of the present invention is to provide a technique for detecting presence of an inoperative nozzle while dispensing with the need to align the ink drop detection device and the print head nozzles with high accuracy.
In order to attain the above and the other objects of the present invention, there is provided a printing apparatus. The printing apparatus comprises a print head, an ink drop detector, a feed mechanism, a detection pulse analyzer, and a nozzle condition determiner. The print head includes a nozzle row having a plurality of nozzles for ejecting ink drops. The plurality of nozzles is aligned in a direction of sub-scanning. The ink drop detector has a light emitter for emitting light and a light receiver for receiving the light emitted by the light emitter. The ink drop detector is configured to generate detection pulses in response to blockage of the light by the ink drops. The feed mechanism is configured to move the print head and/or the ink drop detector in order for the print head and the ink drop detector to move relative to each other. The detection pulse analyzer is capable of: measuring a time interval of two consecutive detection pulses which are detected by the ink drop detector while the print head and the ink drop detector are relatively moving in a constant speed; judging that the two consecutive detection pulses are associated with a same nozzle if the time interval is less than a first threshold value, while judging that the two consecutive detection pulses are associated with two different nozzles if the time interval is greater than the first threshold value; and counting a number of operative nozzles capable of ejecting ink drops based on the judgment. The nozzle condition determiner is configured to determine presence of an inoperative nozzle incapable of ejecting ink drops if the number of operative nozzles is less than a number of test nozzles being subject to the ink drop detection.
In this printing apparatus, an inoperative nozzle can be detected by comparing a specific threshold with a time interval between successive detection pulses, thus making it possible to identify the inoperative nozzle while dispensing with the need to align the ink drop detection device and the print head nozzles with high accuracy.
In a preferred embodiment of the invention, the detection pulse analyzer judges that a missing dot region including at least one inoperative nozzle exists between the two different nozzles associated with the two consecutive detection pulses if the time interval is greater than a second threshold value which is greater than the first threshold value. The nozzle condition determiner further determines presence of an inoperative nozzle based on the judgment of the missing dot region.
The possibility of an inoperative nozzle being overlooked is reduced because the absence of dots is detected based on the logical sum of a detection result related to missing dots and a detection result obtained by determining whether the number of confirmed normally operative nozzles is less than the number of nozzles being tested.
In another preferred embodiment of the invention, the print head comprises a plurality of test nozzle rows. The test nozzle rows are subject to the ink drop detection during a single pass of relative movement of the print head and the ink drop detector. The detection pulse analyzer is capable of: (i) judging that the two consecutive detection pulses are associated with two different test nozzle rows if the time interval is greater than a third threshold which is greater than the second threshold value; (ii) counting a number of test nozzle rows based on the judgment of test nozzle row; (iii) counting a number of operative nozzles in each test nozzle row; and (iv) counting a number of missing dot regions in each test nozzle row. The nozzle condition determiner further determines presence of an inoperative nozzle in an individual test nozzle row if the number of operative nozzles in the test nozzle row is less than the number of test nozzles in the test nozzle row and/or if the missing dot region is detected in the test nozzle row.
Adopting this approach makes it possible, for example, to identify missing dots for each test nozzle rows on the basis of a logical sum of an estimate designed to determine the presence of an inoperative nozzle region and an estimate designed to determine whether the number of confirmed normally operative nozzles is less than the number of test nozzles when a plurality of nozzle rows are tested during a single main scan.
In other preferred embodiment of the invention, the detection pulse analyzer is further capable of: (i) counting a number of operative reference nozzles which are disposed at one of ends of each test nozzle row based on detection signals obtained while only the reference nozzles are ejecting ink drops; (ii) counting a number of operative intermediate nozzles and a number of intermediate missing dot regions, the operative intermediate nozzles and the intermediate missing dot regions being disposed between the reference nozzle and each missing dot regions in each test nozzle rows. The nozzle condition determiner is further capable of: (i) determining that all of the reference nozzles are operative nozzles if the number of operative reference nozzles matches a number of the reference nozzles; and (ii) determining a position of each inoperative nozzle included in each missing dot region in each test nozzle row based on the number of operative intermediate nozzles and the number of intermediate missing dot regions in each test nozzle rows.
Adopting this approach allows successful nozzle operation to be confirmed based on the ejection of ink solely from the end nozzles, making it possible to increase detection accuracy for the end nozzles, whose operation cannot be tested directly by missing dot identification.
In other preferred embodiment of the invention, the detection pulse analyzer counts a number of operative nozzles and a number of missing dot regions which are present before and after each missing dot region. The nozzle condition determiner determines a position of each inoperative nozzle included in each missing dot region based on the number of operative nozzles and the number of missing dot regions present before and after each missing dot regions.
With this approach, a plurality of nozzles are analyzed nozzle by nozzle to identify inoperative nozzles, making it possible, for example, to launch a complementary operating cycle in which dots are formed by alternative nozzles.
In other preferred embodiment of the invention, the feed mechanism is capable of moving the print head and/or the ink drop detector in order for the print head and the ink drop detector to move relative to each other a plurality of times. The plurality of nozzles are divided into a plurality of groups, a selected one of the plurality of groups being subject to testing during one pass of relative movement. The detection pulse analyzer counts a number of operative nozzles during each pass of relative movement. The nozzle condition determiner determines presence of an inoperative nozzle incapable of ejecting ink drops if the number of operative nozzles is less than a number of the test nozzles during each pass of relative movement.
Adopting this approach allows the distance between the nozzles being tested during each main scan to be appropriately increased, making it possible to efficiently prevent situations in which light is blocked by ink drops ejected by certain nozzles when other nozzles are being tested.
The present invention can be implemented as a method or device for detecting nozzle ejection, a computer program for allowing the functions of the method or device to be performed by a computer, a data signal implemented as part of a carrier wave and designed to contain this computer program, or the like.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.